Production of esters

Details Production of esters Production of esters

2000-11-01

2002-11-05

Killos, Paul J. (Department: 1623)

Organic compounds -- part of the class 532-570 series

Organic compounds

Carboxylic acid esters

Reexamination Certificate

active

06476255

ABSTRACT:

The present invention concerns a process for the production of alkyl esters of aliphatic carboxylic acids, in particular methyl propionate, from alkene, carbon monoxide and alkanol.
The production of alkanoic esters and acids by carbonylation of alkenes in the presence of an alkanol or water, as appropriate, is well known. The carbonylation of ethylene using carbon monoxide in the presence of an alcohol or water and a catalyst system comprising a Group VIII metal, e.g. palladium, and a phosphine ligand, e.g. an alkyl phosphine, cycloalkyl phosphine, aryl phosphine, pyridyl phosphine or bidentate phosphine, has been described in numerous European patents and patent applications, e.g. EP-A-0055875, EP-A0489472, EP-A-0106379, EP-A-0235864, EP-A-0274795, EP-A0499329, EP-A-0386833, EP-A-0441447, EP-A-0282142, EP-A-0227160, EP-A-0495547 and EP-A-0495548 and in WO97/03943.
EP-A-0411721 discloses a continuous process for the preparation of an alkyl propionate, which comprises reacting an alkanol in a liquid phase with ethylene and carbon monoxide in a reaction vessel in the presence of a carbonylation catalyst and passing a gas through the reaction vessel, thereby forming a stream of vapour comprising alkyl propionate, gas and unreacted alkanol. The vapour stream is then condensed and the resulting liquid comprises alkyl propionate, alkanol and impurities. The alkanol or alkanol/alkyl propionate azeotropic mixture is distilled off the alkyl propionate product stream and recycled to the reaction vessel.
In the production of methyl propionate from ethylene, carbon monoxide and methanol described in EP-0411721-A, the ratio of methyl propionate product to methanol in the reactor is greater than 50:50 mole %. For example, methanol and other alkanol levels of 18-20% are used. This is not surprising because methanol and methyl propionate form an azeotropic mixture comprising approximately 50 wt % of each component and it may therefore be expected that operating the process such that the composition of the reaction mixture, and thus the product stream, is richer in methyl propionate than methanol, i.e. operating with a composition on the methyl propionate side of the azeotrope, would be beneficial because separation of the methyl propionate product from the residual methanol in the product stream may then be achieved by means of a simple distillation.
Consideration of published information concerning such reactions confirms that it is usual to operate with the ratio of methyl propionate to methanol as described above.
Surprisingly we have found that superior results are obtained if the reaction in the reactor takes place with a concentration of reactant alcohol relative to product ester substantially in excess of that level of alcohol which would form an azeotrope with the ester.
According to the invention a process for the manufacture of an alkyl ester of an aliphatic carboxylic acid comprises the steps of:
a) supplying a first feed stream comprising carbon monoxide and an alkene of formula C
n
H
2n
to a reactor;
b) supplying a second feed stream comprising an alkyl alcohol of formula R—OH, where R is an alkyl group, to the reactor,
c) reacting together said first and second feed streams in the reactor in the presence of a carbonylation catalyst, which catalyses the carbonylation reaction between carbon monoxide and the alkene, to form a product comprising an alkyl alkanoate having the formula C
n
H
2n+1
COOR,
characterised in that the ratio of alkyl alcohol to alkyl alkanoate present in the reactor is greater than the ratio of alkyl alcohol to alkyl alkanoate present in an azeotropic mixture of the alkyl alcohol and alkyl alkanoate.
The carbon monoxide may be used in the presence of other gases which are inert in the reaction. Examples of such gases include hydrogen, nitrogen, carbon dioxide and the noble gases such as argon.
The first feed stream may be fed as a gaseous phase.
The first feed stream and the second feed stream may be fed to the reactor separately or together.
The alkene preferably comprises ethylene. The molar ratio of the ethylene to carbon monoxide in the gaseous phase is preferably greater than 1:1, more preferably at least 3:1, especially from 3:1 to 50:1 and most preferably in the range from 3:1 to 15:1.
Suitable alkyl alcohols include C
1-30
alkanols, optionally substituted with one or more substituents such as halogen atoms, cyano, carbonyl, alkoxy or aryl groups. Suitable alkanols include one or more of methanol, ethanol, propanol, 2-propanol, 2-butanol, t-butyl alcohol and chlorocapryl alcohol. Particularly useful are methanol and ethanol. Additionally or alternatively, polyhydroxyl compounds, such as diols and sugars, may be used. The preferred alkyl alcohol is methanol.
Thus in a preferred form of the invention, we provide a process for the manufacture of methyl propionate comprising the steps of:
a) supplying a first feed stream comprising carbon monoxide and ethylene in the gas phase to a reactor;
b) supplying a second feed stream comprising methanol to the reactor;
c) reacting together said first and second feed streams in the reactor in the presence of a carbonylation catalyst, which catalyses the carbonylation reaction between carbon monoxide and ethylene, to form a product comprising methyl propionate;
characterised in that the ratio of methanol to methyl propionate present in the reactor is greater than 47:53 weight %.
The carbonylation catalyst preferably comprises a combination of palladium or a compound thereof and phosphorous-containing compound. Various phosphine compounds are suitable catalysts and have been described in prior publications, e.g. tertiary phosphines of general formula R
1
R
2
R
3
P wherein R
1
, R
2
and R
3
are each an optionally substituted alkyl or aryl group, e.g. tri-phenyl phosphine. A particularly preferred phosphorous-containing compound is a bidentate phosphine ligand of general formula (R
3
—C)
2
P—L
1
—X—L
2
—P—(C—R
3
)
2
, in which each R is independently a pendant, optionally substituted, organic group through which the group is linked to tertiary carbon atom C; L
1
, L
2
are independently a linking group selected from an optionally substituted lower alkylene chain connecting the respective phosphorous atom to the group X and X is a bridging group comprising an optionally substituted aryl moiety to which the phosphorous atoms are linked on available adjacent carbon atoms. Such a catalyst is described in WO/96/19434.
The pendant optionally substituted organic groups, R, may be independently selected from a wide range of components. Preferably, the pendant groups are optionally substituted lower alkyl, e.g. C
1-8
, and which may be branched or linear. Particularly preferred is when the organic groups, R, when associated with their respective carbon atom form composite groups which are at least as sterically hindering as t-butyl. Steric hindrance in this context is as discussed at page 14 et seq of “Homogeneous Transition Metal Catalysis—A Gentle Art”, by C Masters, published by Chapman and Hall, 1981.
The linking groups, L
1
and L
2
, are independently selected from an optionally substituted, particularly lower alkyl, e.g. C
1
to C
4
, substituted, lower alkylene, e.g. C
1
to C
4
chain. Especially preferred is when both L
1
and L
2
are methylene.
Optional substitution of the aryl moiety, X, may be by other organic groups, e.g. alkyl, particularly C
1-8
, aryl, alkoxy, carbalkoxy, halo, nitro, trihalomethyl and cyano. Furthermore, the aryl moiety may be a fused polycyclic group, e.g. naphthalene, biphenylene or indene.
Examples of suitable bidentate ligands are bis (di-t-butyl phosphino)-o-xylene (also known as 1,2 bis (di-t-butylphosphinomethyl)benzene), bis (di-t-neopentyl phosphino)-o-xylene and bis 1,2(di-t-butyl phosphino)naphthalene. Additionally, the bidentate phosphine may be bonded to a suitable polymeric or inorganic substrate via at least one of the bridging group X, the linking group L
1
or the linking group L
2
, e.g. bis (di-t-butyl phosphino)-o-xylene may be bonded via the xylene group to poly

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